The present invention relates to a new process for the manufacture of urea-formaldehyde resins; its subject matter is more particularly a new process for the manufacture of reactive urea-formaldehyde resins which are stable in storage and give off little formaldehyde.
Urea-formaldehyde resins are known products which are widely employed in the woodworking industry, in particular for the manufacture of particle boards. They are manufactured in a known manner by a condensation of urea and of formaldehyde at a pH of between 4 and 7 and at a temperature close to boiling point; this condensation reaction is preferably carried out in several stages.
The chief disadvantages of urea-formaldehyde resins is that they cause large quantities of free formaldehyde to be given off. Attempts have been made to reduce the free formaldehyde content by using various methods of manufacture; unfortunately, when the aim is to reach particularly low contents of formaldehyde, experience shows that this objective is accompanied both by a decrease in the reactivity and stability of the resins and by a deterioration in the mechanical properties of the finished boards. In order to get rid of free formaldehyde it has also been proposed to employ resins devoid of formaldehyde, particularly resins based on isocyanate solutions. Unfortunately, the problem of formaldehyde is replaced by that is isocyanates, whose effect on main is more harmful and more durable than that of formaldehyde, since the presence of free isocyanate groups has been demonstrated in the particle boards even several years after manufacture.